Liquid crystal display devices are widely used in display means or the like of electronic equipment at present but since liquid crystal display devices are not self-luminous, illuminating means for ensuring visibility at night and in dark places are needed. A planar illumination device has been used as such illuminating means.
As one type of the planar illumination device, a side-light type planar illumination device is widely used. The side-light type planar illumination device comprises a light guide plate with light transmittance, a rod-state light source arranged on a side end face of the light guide plate or one or more point-state light sources as basic elements. As a recent trend, in accordance with an increase in the application of planar illumination devices to small-sized electronic equipment such as personal digital assistants, a planar illumination device of the type provided with a point-like light source capable of simplification of a driving circuit is used. FIG. 15 schematically shows a light guide plate 12 and a plurality of point-like light sources (LED) 14 arranged on a side end face of the light guide plate 12 in a side-light type planar illumination device 10.
In the planar illumination device 10 using the point-like light source 14 shown in FIG. 15, since light irradiated from the LED 14 to the light guide plate 12 has a given directivity, the vicinity of the LED 14 of the light guide plate 12 illuminated by the LED 14 is clearly divided into a bright part A and a dark part B. As a measure for solving a difference between the bright part and the dark part so as to obtain average brightness, an optical diffused reflection pattern 12b such as a fine prism row is provided on an opposite face 12a opposed to the LED 14 of the light guide plate 12 as shown in FIG. 16, an LED 16 in which a part of an armor member is made into a projecting portion 16a in a half-column state is used and a semi-cylindrical recess portion 12c to be fitted with the projecting portion 16a is provided at the light guide plate 12 so as to emit light of an LED chip 17 radially from a slit formed at the projecting portion 16a (See Patent Document 1, for example) as shown in FIG. 17, or moreover, a lamp house 20 covering an LED chip 19 is provided as in an LED 18 shown in FIG. 18 so as to control light directivity by changing a height of the lamp house 20 or an inclination angle of an inclined face 20a (See Patent Document 2, for example). In FIG. 18, reference numeral 22 denotes a translucent resin for sealing the LED chip 19 in the lamp house 20.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 10-199316 ([0023], [0026] to [0028])
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-217459 (Claim 1, FIG. 1)